1. Field of the Invention
The invention is directed to a method for the operation of an internal combustion engine.
2. Description of the Related Art
In addition to a motor, known internal combustion engines also have an exhaust gas aftertreatment system so that exhaust gas formed in the motor of the internal combustion engine during the combustion of fuel can be cleaned in the exhaust gas aftertreatment system. An efficient exhaust gas aftertreatment is required in order to meet increasingly strict exhaust gas limits.
In addition to solid particles, nitrogen oxides (NOx) in the exhaust gas must comply with increasingly strict limits. The nitrogen oxides in the exhaust gas are reduced using catalysts, for example, using selective catalytic reduction (SCR) catalysts. These SCR catalysts utilize ammonia as reductant for converting nitrogen oxides. The ammonia can be generated in an ammonia generator and introduced into the exhaust gas stream. In contrast, it is also possible to introduce an ammonia precursor substance, e.g., an aqueous urea solution, into the exhaust gas stream, which is then converted in the exhaust gas stream to ammonia, carbon dioxide and water vapor. The conversion of the ammonia precursor substance to ammonia in the exhaust gas is typically carried out using a hydrolysis catalyst.
The conversion of nitrogen oxides in the SCR catalytic converter is carried out when nitrogen monoxide exclusively is present in the exhaust gas as expressed by the following equation:4NO+4NH3+O2→4N2+6H2O.
The conversion of nitrogen monoxide in a SCR catalytic converter according to the equation given above takes place relatively gradually. Therefore, in order to accelerate the conversion of nitrogen oxides in the exhaust gas it is already known from practice to position an NO oxidation catalytic converter, particularly a platinum-containing NO oxidation catalytic converter, upstream of the SCR catalytic converter to convert nitrogen monoxide into nitrogen dioxide upstream of the SCR catalytic converter. In this case, when nitrogen dioxide is also present in the exhaust gas in addition to nitrogen monoxide, the conversion of the nitrogen oxides in the catalytic converter takes place according to the following equation:NO+2NH3+NO2→2N2+3H2O.
The conversion of nitrogen oxides according to the formula cited above in the presence of nitrogen monoxide and nitrogen dioxide in the SCR catalytic converter takes place faster than the pure conversion of nitrogen monoxide in the SCR catalytic converter.
Accordingly, the rapidity of the conversion of nitrogen oxides contained in the exhaust gas in the SCR catalytic converter depends on the proportion of nitrogen dioxide in the exhaust gas. However, the use of a separate NO oxidation catalytic converter for converting the nitrogen monoxide into nitrogen dioxide upstream of a SCR catalytic converter is disadvantageous because this increases expenditure on apparatus and, therefore, the costs for an internal combustion engine.